1. Field of the Invention
The present invention relates to a switching power supply device that interrupts electric power supplied from an input power supply and that converts electric power via an inductor to output a predetermined direct-current voltage.
2. Description of the Related Art
Generally, performance indicia of a switching power supply device include a harmonic characteristic and a power factor characteristic. The harmonic characteristic is a function of suppressing flow of harmonic current from the switching power supply device to its input power supply line. An upper limit of the harmonic current is regulated so as not to adversely influence another device. In addition, the power factor characteristic is a power factor when an input is viewed from the switching power supply device. The power factor is preferably relatively high in order to reduce a loss in an electric power system.
In the related art, the switching power supply device described in International Patent Application Publication No. W02005/074113 or Japanese Unexamined Patent Application Publication No. 2000-116126 has been provided.
FIG. 12 shows an example of the configuration of a switching power supply device of International Patent Application Publication No. W02005/074113. In FIG. 12, a first switch circuit (S1) is defined by a parallel circuit including a first switch element (Q1), a first diode (D1) and a first capacitor (Cds1), and a second switch circuit (S2) is defined by a parallel circuit including a second switch element (Q2), a second diode (D2) and a second capacitor (Cds2).
Vin is an input power supply, and is a commercial alternating current power supply. Vin is rectified by a rectifier diode (Da) via an EMI filter (EMI-F). T is a transformer. The transformer T defines a closed loop including a primary coil (Lp), a first inductor (Lr), the first switch circuit (S1) and the rectifier diode (Da). The transformer T connects a series circuit of a second switch circuit (S2) and a capacitor (Cr) to a series circuit of the primary coil (Lp) and the first inductor (Lr) in parallel with each other.
A rectifying/smoothing circuit defined by a rectifier diode (Ds) and a smoothing capacitor (Co) is provided for a secondary coil (Ls) of the transformer (T). A capacitor (Cs) is connected in parallel with the secondary-side rectifier diode (Ds).
A feedback circuit (FB1) detects a voltage (Vo) output from the rectifying/smoothing circuit (RS) to an output terminal (OUT), and performs feedback control so that the voltage (Vo) is stable. A first switching control circuit (SC1) inputs a voltage generated at a drive coil (Lb1) to control an off timing of the first switch element (Q1), thus controlling an on period of the first switch element (Q1).
A second switching control circuit (SC2) inputs a voltage generated at a drive coil (Lb2) to control an off timing of the second switch element (Q2), thus controlling an on period of the second switch element (Q2). In addition, one end of a second inductor (Li) is connected to a connecting point of the first switch circuit (S1) and the second switch circuit (S2), and the other end of the second inductor (Li) is connected to a third diode Di. In addition, both ends of a fourth diode (Dc) are respectively connected to a connecting point of the second switch circuit (S2) and a fifth capacitor (Cr) and a connecting point of the third diode (Di) and the second inductor (Li).
A fourth capacitor (Ci) is connected between a connecting point of the first switch circuit (S1) and a third capacitor (Ca) and one end of the first inductor (Lr).
The switching control circuits (SC1 and SC2) are respectively connected to the first and second switch circuits (S1 and S2). A fourth diode (Db) is connected between the input-side rectifier circuit (Da) and the fourth capacitor (Ci).
The switching control circuit (SC1) includes a transistor (Tr1), a delay circuit (DL1) and a time-constant circuit (TC1) connected between the gate and source of the first switch element (Q1). The delay circuit (DL1) is defined by a series circuit, including a capacitor (Cg1) and a resistor (Rg1), and an input capacitance (not shown) of the switch element (Q1). The first switch element (Q1) turns on by a voltage induced by the drive coil (Lb1), and a turn-on timing of Q1 is delayed by the delay circuit (DL1).
The time-constant circuit (TC1) includes a capacitor (Ct1) and an impedance circuit including a resistor (Rt1), a diode (Dt1), and a phototransistor (Pt1) of a photocoupler. The time-constant circuit (TC1) and the transistor (Tr1) control an on period of the first switch element (Q1).
The second switching control circuit (SC2) also has a similar configuration to that of the first switching control circuit (SC1), and operates similarly.
The feedback circuit (FB1) is connected to the phototransistor (Pt1) of the photocoupler of the first switching control circuit (SC1). The feedback circuit (FB1) detects a voltage (Vo) output from the rectifying/smoothing circuit (RS) to the output terminal (OUT), and performs feedback control such that the voltage (Vo) is stable. A second feedback circuit (FB2) detects an input voltage (Vi) of the fourth capacitor (Ci), and performs feedback control such that the on period of the second switch element (Q2) is controlled so that the input voltage (Vi) does not increase beyond a predetermined value under a light load.
FIG. 13 shows an example of the configuration of a switching power supply device shown in FIG. 6 of Japanese Unexamined Patent Application Publication No. 2000-116126. In FIG. 13, a series circuit including a reactor (L1) and a diode (D1) is connected to one end of an output terminal of a diode bridge (DB) that rectifies a commercial power supply (VAC), a smoothing capacitor (C1) is connected between the diode (D1) and the other end of the output terminal of the diode bridge (DB), and a switch element (S1) is connected between a connecting point of the reactor (L1) and the diode (D1) and a connecting point of the diode bridge (DB) and the capacitor (C1).
In addition, a partial voltage resonance DC/DC converter is provided so that a series circuit including a primary coil of an isolation transformer (TR1) and a main switch element (S2) is connected in parallel with a positive electrode terminal and negative electrode terminal of the smoothing capacitor (C1), a series circuit including a capacitor (C5) and an auxiliary switch element (S3) is connected between a connecting point of the isolation transformer (TR1) and the main switch element (S2) and the positive electrode terminal of the smoothing capacitor (C1), and capacitors (C4 and C3) are respectively connected in parallel with the main switch element (S2) and the auxiliary switch element (S3). In addition, diodes (D3 and D4) are respectively connected in antiparallel with the main switch element (S2) and the auxiliary switch element (S3). That is, the diodes (D3 and D4) are respectively connected in parallel with the main switch element (S2) and the auxiliary switch element (S3) and current flows through the diodes (D3 and D4) in a direct opposite to the direction in which current flows through the switch element (S2) and the auxiliary switch element (S3). In addition, a series circuit including the diode (D2) and the capacitor (C2) is connected between both ends of a secondary coil of the isolation transformer, and both ends of the capacitor (C2) define a direct-current output terminal.
However, as described in Claim 19 of International Patent Application Publication No. W02005/074113, the switching power supply device enters an intermittent oscillation mode under a light load state or a no load state. In the intermittent oscillation mode, an oscillation period and an interruption period are periodically repeated.
In this case, the frequency of intermittent oscillation falls within an audible frequency range, and may possibly cause noise.
In addition, there is a problem in that a ripple of an output voltage increases because of the intermittent oscillation.
In addition, in Japanese Unexamined Patent Application Publication No. 2000-116126, a power-factor correcting circuit and the partial resonance DC/DC converter are separately controlled. Thus, there is a problem in that a beat interference may occur because of interference between different switching frequencies or a control circuit becomes complex because a protection circuit and a control circuit for overcurrent and their voltage sources are required.